Forty-three consecutive patients with mechanical valve prostheses underwent transthoracic and transoesophageal echocardiography for suspected thrombolic prosthetic valve dysfunction. The results of these investigations were compared with those of cineradiography and the clinical outcome. The diagnosis of thrombosis was retained in 11 of the 43 patients (10 mitral and 1 aortic valve prostheses). The transthoracic Doppler echo was clearly abnormal in 6 of the 11 cases. Transoesophageal echo was of essential value in all cases but one, showing abnormal movement of the mobile element and/or a paraprosthetic thrombus. The cineradiography gave false negative results in 6 cases. In conclusion, these cases underline the undeniable value of transoesophageal echocardiography in occlusive or non-occlusive thrombosis of a mitral valve prosthesis. This investigation should be undertaken whenever there is the least suspicion of thrombosis of the prosthesis.